Steady-state coherent magnetic resonance imaging (MRI) involves fast sequences that yield an excellent signal to noise ratio (SNR) and a unique contrast. TrueFISP (True fast imaging with steady-state precession), which is also known as bSSFP (balanced steady-state free precession), is one example fast steady-state coherent MRI technique. Steady-state coherent MRI has historically been sensitive to variations in the B0 field. Variations in the B0 field may lead to phase accumulation in steady-state coherent MRI.
Phase accumulation may also be referred to as the “off-resonance angle” that causes spins in some locations in an object being imaged to become parallel with a radio frequency (RF) field employed during the steady-state coherent MRI. When the spins become parallel to the field, those spins will not experience desired excitation due to the pulse sequence. Spins that are not excited will not produce the correct level of nuclear magnetic resonance (NMR) signal from which a magnetic resonance (MR) image can be formed. Therefore, the regions where the spins are not excited will appear as dark bands in an MR image. More particularly, continuous signal voids may appear in an MR image at locations where the off-resonance angle is an odd-integer multiple of π. The continuous signal voids may lead to the dark bands, which in turn may obstruct underlying pathology, which is undesirable.
Conventional attempts to address phase accumulation and the off-resonance angle issue have included shimming and multi-dimensional pulses. However, neither of these approaches has provided desired mitigation of banding effects associated with phase accumulation. Shimming is a mature technology and recent efforts have tended to produce only incremental improvements. Additionally, shimming may be limited in its ability to adapt to certain B0 situations. Multi-dimensional pulses have also conventionally been employed to mitigate phase accumulation. Tailored multi-dimensional pulses may mitigate some issues associated with B0 inhomogeneity. However tailored multi-dimensional pulses tend to be very long in duration, which defeats some of the purpose of fast sequences like those associated with steady-state coherent MRI (e.g., TrueFISP).